Thank you. Lingua::EN::Contraction is good enough for for my purpose.(My work consists in comparing two English sentences and check if they are the same, but first I needed to normalize them, so this module does this, which is more than OK)

Maybe the inverse ("what's" to "what is") is not possible because there are some words like person's which doesn't (always) mean person is.(I'm not a native English speaker, so I may be wrong about this)